One time programmable read only memories ("OTPROMs") store individual binary bits of information in memory cells. Given their "read only" nature, once programmed, these memories are essentially limited to being read only devices. At this point, these devices cannot be reprogrammed, and thus, no new information can be entered. To insure the viability of these devices, the binary data stored within the OTPROM is non-volatile. The non-volatility of the data enables the preservation of the data when power is removed.
PROMs comprise a plurality of word lines, and a plurality of parallel bit lines. The bit lines are perpendicular to the word lines and the general area of each intersection of a word line and a bit line, along with any associated circuitry, constitutes a single memory cell. Normally, the presence or absence of a conducting semiconductor device, diode or transistor, connecting the word line and the bit line determines the binary "1" or "0" state of the memory cell. The absence of a conducting semiconductor can result from (1) the absence of a complete transistor or diode, or (2) the presence of a high resistance in series with the semiconductor.
Fundamentally, the absence of a conducting semiconductor can be achieved by several means known in the art. One known approach is usage of fuse-type coupling. By linking the semiconductor through a fuse, the absence of conductivity is achieved when a sufficient voltage is applied so as to disintegrate the fusible link.
Another known strategy has been to employ an antifuse link having a thin dielectric layer. Utilizing this approach, each conducting semiconductor is disassociated with the memory prior to programming. However, by applying appropriate voltages to the antifuse, its dielectric layer can be disabled, thereby allowing the semiconductor to conduct.
Several problems remain with respect to the PROM. Industry demands a PROM cell that can be programmed at high voltages over a shorter period of time. Further, there still remains a need for a PROM cell that can more easily be isolated from the remaining memory array. Moreover, a demand exists for a PROM cell which is more simple to manufacture, and is easier to shrink, overcoming problems in alignment, sizing, and matching materials.